1. Field
This invention involves an instrument for precisely measuring visual function in the fovea centralis (macula) area of the retina, and in particular, pathologic incremental changes in foveal (macular) vision caused by age related macular degeneration.
2. Problem
The fovea centralis or macula is the small central area (approximately 0.3 millimeters in diameter) of the retina that exhibits maximum visual acuity.
Many people in the senior-citizen time frame are affected by a pathological process that causes degeneration of the fovea and loss of visual function--a process typically referred to as Age-Related Macular Degeneration (AMD). AMD progresses from the loss of foveal sensitivity (i.e. reduced intensity, blur, fragmentation or other distortion), to the eventual complete loss of foveal vision. Due to increasing life-spans, more people are now vulnerable to AMD, which has become a significant source of visual disability.
AMD causes the fovea to degenerate into irregular segments that lose visual function at independently varying rates. AMD is the only foveal condition that characteristically degenerates into irregular segments of varying visual loss. Other causes of abnormalities in foveal vision result in substantially uniform loss over the whole foveal area.
Evaluating foveal degeneration caused by AMD requires detecting foveal segmentation and measuring incremental abnormalities or changes within the segments. Such evaluation is required to detect the onset and monitor the progression of foveal degeneration, thereby facilitating early and efficacious medical treatment. Moreover, without precise evaluation of foveal degeneration, judging the efficacy of experimental treatments is highly problematic.
Accordingly, the need exists for a foveal vision evaluation instrument capable of precisely evaluating foveal segmentation and degeneration, including measuring incremental abnormalities and changes in foveal visual function, as an aid to detection and treatment of AMD.
3. Prior Art.
Existing instruments for testing vision are for measuring and correcting refractive errors associated with the loss of visual acuity. AMD symptoms can be identified by observing the fovea with an ophthalmoscope; however, the observed characteristics of the diseased fovea do not correlate with visual impairment or changes in visual impairment, and cannot be used to evaluate the degeneration of foveal vision.
These instruments are not effective in evaluating the onset or progression of AMD because they cannot (a) isolate on the fovea, (b) detect foveal segmentation, or (c) monitor incremental abnormalities or changes in foveal vision. That is, they are not able to identify or locate abnormal areas throughout the foveal field of vision--abnormalities that can be localized within certain foveal segments and manifested as reduced intensity, blurring and/or fragmentation.
For example, at the standard projection distance of twenty feet, the foveal field of vision is a central area on the screen approximately fifteen centimeters in diameter. None of the available instruments is able to project an image array of letters or symbols within the foveal field of vision in order to make incremental changes in the size, presence or contrast of the image elements so as to identify and document foveal segmentation and incremental abnormalities in foveal vision.
Despite advances in knowledge of the physiology and pathology of the retina in general, practically no detailed information about the pathologic physiology of foveal degeneration has been revealed beyond the observation that victims of AMD experience degeneration in foveal vision unpredictably over a period of weeks to years until foveal vision is lost. Indeed, because of the lack of instruments for evaluating foveal vision, knowledge about foveal pathology must primarily be derived from specimen eyes.